1. Field of the Invention
The present invention relates to a measurement device for measuring the strain that appears in an object to be measured, such as a structure, and/or temperature of the object, by using an optical fiber. More particularly, it relates to an improvement in a measurement of the strain that appears in an object to be measured and/or the temperature of the object, which makes it possible to measure them with a very high spatial resolution.
2. Description of the Prior Art
Referring now to FIG. 25, there is illustrated a block diagram showing the structure of a prior art measurement device for measuring the strain that appears in an object to be measured and/or temperature of the object to be measured, by means of an optical fiber, by using a technique as disclosed in Japanese Patent Application Publication (KOKAI) No. 3-120437, for example. In the figure, reference numeral 1 denotes an object to be measured, 2 denotes an optical fiber, 3 denotes a pump light source for emitting and injecting discontinuous pump light into an end (first end) of the optical fiber 2, 4 denotes a probe light source for emitting and injecting continuous probe light into another end (second end) of the optical fiber 2, 5 denotes a light intensity measurement unit (or light detector) for sampling the light intensity output light emitted from the first end of the optical fiber 2 and for furnishing light intensity data, 6 denotes a multiplexer/branching coupler for causing the output light from the optical fiber 2 to branch to the light intensity measurement unit 5, 7 denotes a filter located between the multiplexer/branching coupler 6 and the light intensity measurement unit 5, for transmitting Brillouin scattered light included in the output light, and 8 denotes a computation unit for computing the strain that appears in a predetermined zone defined in the optical fiber 2, or the temperature of the predetermined zone, based on the light intensity data from the light intensity measurement unit 5.
In operation, the pump light source 3 injects discontinuous pump light with a certain frequency into the first end of the optical fiber 2 first while the probe light source 4 injects continuous probe light into the second end of the optical fiber 2. While the discontinuous pump light travels through the optical fiber 2, the discontinuous pump light and the continuous probe light overlap one another at a certain position of the optical fiber 2. If the frequency of the probe light agrees with any one of the frequencies of scattered light lines, such as Brillouin scattered light lines, caused by the pump light, the scattered light is amplified enough to be easily detected. The multiplexer/branching coupler 6 causes output light emitted from the first end of the optical fiber 2 to branch to the filter 7. The filter 7 then transmits only the amplified probe light, which will be referred to as Brillouin scattered light, included in the output light from the multiplexer/branching coupler6. The light intensity measurement unit 5 samples the light intensity of the Brillouin scattered light.
Such a process of measuring the Brillouin scattered light is repeated with different frequencies of the continuous probe light, and the computation unit 8 then receives and stores intensity data of the measured light intensities of the sampled Brillouin scattered light associated with a predetermined zone defined in the optical fiber 2, which lies within a certain band of frequencies corresponding to the range in which the frequency of the probe light has been scanned. Thus the computation unit 8 can get a spectrum of the Brillouin scattered light associated with the predetermined zone. The computation unit 8 then determines the frequency of the sampled Brillouin scattered light with the largest light intensity as a center frequency of the Brillouin scattered light associated with the predetermined zone defined in the optical fiber, and computes a frequency shift in the center frequency computed from a reference center frequency that was measured for the optical fiber 2 with no strain. The computation unit 8 thus can compute the strain that appears in the predetermined zone defined in the optical fiber based on the frequency shift computed.
A problem with the prior art measurement device constructed as above is that it cannot measure the strain that appears in an object to be measured or temperature of the object to be measured with a fine (or high) spatial resolution. To be more specific, since an optical fiber having high transparency to light with a wavelength of about 1.5 .mu.m is generally used, Brillouin scattered light is generated with a bandwidth of about 50 MHz when such light with a high intensity is incident on the optical fiber. As a result, even if discontinuous pump light with a time duration less than 20 nsec is injected into the optical fiber under the circumstances, Brillouin scattered light is generated with a bandwidth of about 50 MHz. In other words, Brillouin scattered light has been widened over a time period of 20 nsec. Accordingly, pump light with a time duration less than 20 nsec will generate scattered light similar to that generated by pump light with a time duration of 20 nsec, and therefore the prior art measurement device can only achieve a spatial resolution around the length (=about 2 m=2.times.10.sup.8 m/s.times.10.times.10.sup.-9, assuming that the speed of light is 2.times.10.sup.8 m/s), which is determined by the duration of the discontinuous pump light.